Tax expert and UK government adviser John Whiting said he was sceptical that the amount hidden was so large.

Mr Whiting, tax policy director at the Chartered Institute of Taxation, said: “There clearly are some significant amounts hidden away, but if it really is that size what is being done with it all?”

Mr Henry said his $21tn is actually a conservative figure and the true scale could be $32tn. A trillion is 1,000 billion.

Mr Henry used data from the Bank of International Settlements, International Monetary Fund, World Bank, and national governments.

His study deals only with financial wealth deposited in bank and investment accounts, and not other assets such as property and yachts.

The report comes amid growing public and political concern about tax avoidance and evasion. Some authorities, including in Germany, have even paid for information on alleged tax evaders stolen from banks.

The group that commissioned the report, Tax Justice Network, campaigns against tax havens.

Mr Henry said that the super-rich move money around the globe through an “industrious bevy of professional enablers in private banking, legal, accounting and investment industries.

“The lost tax revenues implied by our estimates is huge. It is large enough to make a significant difference to the finances of many countries.